The present invention relates to an oxidation catalyst of a gaseous compound, particularly a catalyst for promoting an oxidation reaction of a gaseous compound by oxygen ions supplied through a solid electrolyte under an environment where no oxygen molecules are present, more particularly relates to an oxidation catalyst of a gaseous compound which can be suitably used for a solid electrolyte fuel cell.
A solid electrolyte fuel cell can promise a higher efficiency of power generation compared with thermal power generation etc., so is being widely researched at the present.
The applicant proposed the solid electrolyte fuel cell shown in FIG. 1 in Japanese Unexamined Patent Publication (Kokai) No. 2000-348736.
The solid electrolyte fuel cell shown in FIG. 1 has a solid electrolyte element 16A comprised of an oxygen ion conducting type solid electrolyte substrate 10 and electrodes 12 and 14a formed on the two surfaces of this solid electrolyte substrate 10.
The solid electrolyte substrate 10 is comprised of a zirconia (YSZ) fired body stabilized by 8 mol % of yttria (Y2O3)
One electrode 12 is comprised of lanthanum strontium manganoxide [(La0.8Sr0.15)0.90MnO3], is supplied with oxygen, and acts as a cathode.
The other electrode 14a is substantially comprised of a porous platinum layer, is supplied with methane gas as a fuel, and acts as an anode.
The oxygen (O2) supplied to the cathode 12 is ionized at the interface between the cathode 12 and solid electrolyte substrate 10 and becomes oxygen ions (O2xe2x88x92) which are conducted through the solid electrolyte substrate 10 to the anode 14a. These oxygen ions (O2xe2x88x92) react with the methane (CH4) gas supplied to the anode 14a to generate water (H2O), carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO). At the time of this reaction, electrons are emitted from the oxygen ions at the anode 14a, so a potential difference is caused between the cathode 12 and the anode 14a. If the cathode 12 and the anode 14a are electrically connected by an output line 18, the electrons of the anode 14a flow through the output line 18 in the direction of the cathode 12 (direction of arrow mark in figure) and electricity can be output from the solid electrolyte fuel cell.
The anode 14a is comprised of a porous platinum layer 22a formed on one surface of the solid electrolyte substrate 10 and an oxidation catalyst layer 22b formed on the outer surface carrying metal oxide particles comprised of PdCoO2. The oxidation catalyst layer 22b promotes the oxidation reaction between the oxygen ions (O2xe2x88x92) and methane at the anode 14a. 
The solid electrolyte fuel cell shown in FIG. 1, by using the above oxidation catalyst, can improve the power generating characteristic compared with conventional solid electrolyte fuel cells using cermet particles comprised of nickel (Ni) and nickel oxide (Nio) as an oxidation catalyst.
In the solid electrolyte fuel cell shown in FIG. 1, the PdCoO2 metal oxide particles used as the oxidation catalyst were ones obtained by double decomposition or high temperature pressure synthesis. To obtain the PdCoO2, in double decomposition, PdCl2 and CoO are made to react for double decomposition under a high temperature and high pressure, while with high temperature pressure synthesis, PdO and CoO are sealed in a platinum tube and heated.
With each of these methods, the metal oxide particles obtained are large and uneven in size, so have been pulverized to make the particles finer and then sieved to obtain a uniform particle size. This is because the finer the metal oxide particles making up the oxidation catalyst, the higher the oxidation action obtained.
There are however the problems that there are limits to how fine the particles can be made by pulverization and that the cost of manufacture of the oxidation catalyst rises due to the need of the pulverization process.
The present invention has as its object the provision of an oxidation catalyst of a gaseous compound superior in oxidation action comprised of fine metal oxide particles.
To achieve the above object, the oxidation catalyst of a gaseous compound of the present invention comprises a catalyst for promoting an oxidation reaction of a gaseous compound by oxygen ions supplied through a solid electrolyte under an environment where no oxygen molecules are present, characterized by
comprising metal oxide particles produced by firing a precipitate comprised of a mixture of two types of metal hydrates obtained by coprecipitation from a mixed solution of two types of metal salts of different metal positive ions dissolved together, the composition of the metal oxide particles being expressed by the formula:
ABO2
where, A is one element selected from the group consisting of Pd, Pt, Cu, and Ag,
B is one element selected from the group consisting of Co, Cr, Rh, Al, Ga, Fe, In, Sc, and Tl.
Typically, by firing a precipitate comprised of a mixture of Pd(OH)2 and Co(OH)2 obtained by coprecipitation from a mixed solution of PdCl2 and CoCl2xc2x76H2O dissolved together, it is possible to obtain fine metal oxide particles comprised of PdCoO2.
By the conventional double decomposition or high temperature pressure synthesis, depending on the mixed state or particle size of the starting material, there is an effect on the particle size of the metal oxide particles finally obtained and as a result an effect on the catalytic action of the oxidation catalyst.
The precipitate obtained by mixing two types of metal hydrates obtained by coprecipitation is comprised of fine particles of an even particle size, so the metal oxide particles obtained by firing the precipitate are also fine and uniform in particle size. By using the metal oxide particles as an oxidation catalyst, it is possible to obtain a high oxidation action.